Proton conducting membrane for fuel cells

ABSTRACT

An ion conducting membrane comprising dendrimeric polymers covalently linked into a network structure. The dendrimeric polymers have acid functional terminal groups and may be covalently linked via linking compounds, cross-coupling reactions, or copolymerization reactions. The ion conducting membranes may be produced by various methods and used in feul cells.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 10/105,203 filed Mar. 25, 2002, which claims priority to U.S.Provisional Application No. 60/278,979 filed Mar. 27, 2001, the entiredisclosures of which are incorporated herein by reference.

GOVERNMENT INTERESTS

This invention was made with Government support under Contract No.W-31-109-ENG-38 awarded by the Department of Energy. The Government hascertain rights in this invention.

FIELD OF THE INVENTION

This invention relates to proton or hydronium ion conducting membranesbuilt up from dendrimers that are covalently linked together to formmacromonomers and larger nanoscale polymer objects. The invention alsorelates to fuel cells made from the membranes.

BACKGROUND OF THE INVENTION

The proton or hydronium ion conducting membranes that are currently usedin polymer electrolyte membrane fuel cells (PEMFCs) and direct methanolfuel cells (DMFCs) are typically linear or comb polymers that havesulfonic, carboxylic, or phosphonic acid groups located at the end ofshort branches extending from a fluorocarbon or hydrocarbon polymerbackbone. In a fuel cell, these membranes are coated with an anode and acathode on opposite sides, and then stacked between bipolar plates. Inthe polymer electrolyte membrane fuel cell, hydrogen gas and air arepassed over the anode and cathode respectively to generate electricity.In the direct methanol fuel cell, a dilute solution of methanol is usedas fuel. In conventional membranes the terminal acid groupsself-organize into hydrophilic domains that are delineated by thehydrophobic fluorocarbon or hydrocarbon backbone of the polymer and formchannels that contain water. The ionic conductivity of these membranesdepends on the number of acid sites per unit weight of the polymer, andon the ability to maintain the proper water content within the polymermembrane. More acid sites provide more protons or hydronium ions toenhance the ionic conductivity. Because proton transport is facilitatedby water, higher water content within the membrane also translates intohigher conductivity. However, if the water content of the membranebecomes too high, it will lead to swelling of the membrane and asubsequent loss of mechanical strength. Conversely, if the water contentof the membrane is too low, then this will lead to a loss in ionicconductivity. This creates a problem when trying to operate thesemembranes at temperatures above 100° C., which is where one would liketo operate a fuel cell to limit the negative effects of carbon monoxideon the fuel cell performance. As the temperature is increased above 100°C., the water content in the polymer decreases and the conductivitydrops off drastically. In addition, these membranes suffer from methanolcrossover, the diffusion of methanol through the membrane with the waterpresent in the membrane, when used with DMFCs. The performance of DMFCsis severely limited by methanol crossover through the polymerelectrolyte membrane, where as much as 40% of methanol can be lost as itdiffuses from the anode to the cathode compartment of the fuel cell.

Methanol crossover arises because methanol is readily transported fromthe anode to the cathode through the hydrophilic channels within theproton-conducting membrane by bulk diffusion. Methanol is alsotransported as part of the solvation shell around the proton(electroosmotic drag). The ratio of methanol molecules to watermolecules solvating the proton is identical to their concentration ratioin solution. Thus, electroosmotic drag increases as the methanolconcentration in the fuel increases. Electroosmotic drag of methanol isresponsible for the sharp decline in DMFC performance at elevatedmethanol concentrations. Elimination of electroosmotic drag would allowthe use of higher concentrations of methanol, which would increase thefuel efficiency of the DMFC power source.

The problem of methanol crossover is further complicated because thecurrent efforts in membrane development remain focused on increasingpower densities and mechanical durability while decreasing theacid-equivalent weight (EW) and membrane thickness. The EW number is agood measure for the ionic conductivity of the polymer. It is defined asthe molecular weight of polymer per acid group. The lower the EW number,the higher the acid density on the polymer and the higher the protonconductivity. For example, an EW of 1100 means that for every mole ofsulfonic acid there are 1100 grams of fluorocarbon polymer backbone.Commercially available NAFION (perfluorosulfonic acid polymer) istypical of the perfluorinated ionomer membranes used in practical fuelcells. The membrane consists of a fluorinated polymer backbone withstrongly acidic functional groups attached to the polymer chain. NAFIONmembranes have relatively high EWs and low specific conductivities (1100and 0.081 Ω⁻¹ cm⁻¹, respectively, for NAFION 117). For comparison, thesimilar Dow® membranes are somewhat better, with EWs of 800 and 850 forspecific conductivities of 0.20 and 0.12 Ω⁻¹ cm⁻¹, respectively.Therefore, decreasing the EW results in markedly higher protonconductivities while a thinner membrane reduces ionic resistance. Thesefactors together yield an increase in the DMFC power density and overallmembrane performance. However, reductions in the EW have beenaccompanied by an increase in methanol crossover, and thinner membranestend to exhibit reduced durability with an increased risk of methanolcrossover.

The elimination or reduction of methanol crossover in a fuel cell woulddecrease the size of a fuel cell stack needed for a mobile power source,decrease the loss of fuel, and allow the use of higher concentrations ofmethanol in the fuel cell. With presently used proton conductingmembranes, any decrease in methanol permeability also correlates withdecreased proton conductivity.

Thus, a need exists for a durable proton conductive membrane having alow EW and high resistance to methanol crossover.

SUMMARY OF THE INVENTION

The present invention provides proton and hydronium ion conductingpolymer electrolyte membranes that use dendrimeric polymers asmacromonomers that can be covalently assembled into network structuresand nanoscale objects. Such membranes are able to decrease methanolcrossover without increasing the EW of the membranes by decouplingprotonic conductivity and water content. By utilizing dendrimers havingacid functional terminal groups, the membranes of this invention obtaina higher number of acid sites per unit weight of polymer than membranesmade from linear and comb polymers. In addition, the use of dendrimermembranes allows for control of membrane pore size to limit the watercontent in the pores. This allows for operation at higher temperaturesand reduces methanol crossover in direct methanol fuel cells. As isgenerally the case for cross-linked polymers, membranes fabricated inthis manner should also be thermally stable well above the glasstransition temperature for a given dendrimer.

The membranes have many potential uses, including, but are not limitedto, ionic conducting membranes for polymer electrolyte membrane fuelcells (PEMFC) and membranes for direct methanol fuel cells (DMFC). Thedendrimers are prepared from several stepwise syntheses that providecontrol over the dendrimer size and allow for tuning the chemicalfunctionality on the terminus of the dendrimer. Using this approach, theion transport properties of the membranes can be controlled.

The membranes of the present invention offer several advantages over theprior art. The pore structure of the dendrimer membranes results fromthe void volume that exists between adjacent dendrimers, and is afunction of how the dendrimers pack together and the size of themolecular linker used. By controlling the size of the dendrimers and thelinking groups, one can control the size of the pores thereby limitingthe water content in the pores. Also, by utilizing hyperbranchedpolymers, a higher number of acid sites per unit weight of polymer canbe obtained, increasing the ionic conductivity compared to a linearpolymer. In addition, the network structure formed by covalently linkingdendrimers together will make the ionomer membrane resistant toswelling. In one embodiment of the invention the membrane will have athickness of between about 0.02 and about 0.2 millimeters.

Another advantage offered by the dendrimers of the present invention isthat proton or hydronium ion transport may proceed on the surface of thespherical dendrimers, much like grain boundary diffusion in solids. Thisdrastically decreases the dependence of the ionic transport on the watercontent. Given the higher thermal stability and anticipated ionictransport mechanism, these membranes should be useful in highertemperature regimes (100-200° C.).

The membranes are formed by covalently linking dendrimers having acidfunctional terminal groups. The dendrimers can be linked through amidoor imido bridges, as well as other bridging groups, to form a networkpolymer that is cast into a membrane. This approach leads to a randomcovalent network. Alternatively, synthetic strategies can be used inorder to cross-couple or copolymerize dendrimers via specific terminalfunctional groups other than the acid functional groups. Numeroussynthetic methods are available to selectively cross-couple specificfunctional groups, such as halide or hydroxyl groups, to form stablecovalent links between modified dendrimers. This offers the advantage ofdirecting the assembly of the dendrimers as macroscopic building blocksfor an ionomer membrane, and it has implications for controlling thepore microstructure of the membrane on a molecular level.

One example of a suitable dendrimer is the Frechét-type dendrimer. Thearyl-ether linkage structure of Frechét-type dendrimers is robust andleads to a membrane that exhibits significant resistance toward chemicalattack within the highly corrosive fuel cell environment.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 shows an example of a Frechét-type dendrimer.

FIG. 2 shows a representation of a portion of a membrane according tothe present invention comprised of spherical dendrimers covalentlylinked with diamine compounds.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides ion-conducting membranes based oncovalent assemblies of highly functionalized and covalently cross-linkedhyperbranched dendrimers. The present invention also provides fuel cellsmade from the membranes. The dendrimers are terminated with acidfunctionalities and can be covalently bound into a dendrimeric networkby a variety of dendrimer-linking reactions. The covalently-linkednetworks can be cast into membranes which, in turn, can be incorporatedinto fuel cells.

Dendrimers having poly (aryl ether) linkages are well suited for use inthe present invention. Frechét-type dendrimers, which contain poly (arylether) linkages, are particularly suited for use in constructing themembranes of this invention. FIG. 1 shows an example of a Frechét-typedendrimer. This dendritic polyether macromolecule can be made by aconvergent synthesis using 3,5-dihydroxybenzyl alcohol as its basicbuilding block and with carboxylic acid groups as chain ends. Methodsfor making Frechét-type dendrimers are well known in the art. Forexample, one route to this dendrimer has been described by Hawker et al.J. Chem. Soc. Perkin Trans. (1993) 1, 1287, which is incorporated hereinby reference. Briefly, two equivalents of methyl-p-bromomethylbenzoateare reacted with 3,5-dihydroxybenzyl alcohol in the presence of18-crown-6 and potassium carbonate in refluxing acetone. The resultingfirst generation alcohol is converted to the bromide by reaction withcarbon tetrabromide/triphenylphosphine. Reaction of two equivalents ofthis bromide with 3,5-dihydroxybenzyl alcohol leads to the secondgeneration dendrimer. These steps are repeated to get to generation 3,then 4 dendrimers. At generation 4, two equivalents of brominatedgeneration 4 dendrimer are reacted with 4,4′-dihydroxy biphenyl toprovide a macromolecule. The methyl ester groups are then converted byreaction with excess potassium hydroxide to provide the carboxyterminated dendrimer.

The resulting macromolecule exhibits a polymer architecture that isperfectly hyperbranched, and is equivalently envisioned as being aunimolecular micelle or globular amphiphile. In fact, this type ofdendrimer, especially at high molecular weights, closely approximates arigid sphere whose structural integrity is largely independent of thenature of the solvent and its concentration in that solvent. Thiscontrasts markedly with linear polymers, which are random coils whosesize and shape vary as a function of solvent and concentration. As such,this type of dendrimer is well suited for use as a structurallydiscrete, macromolecular building block for larger, nano- and mesoscaleobjects such as organic films, composite materials and membranes. Theinternal aryl-ether linkage structure of this dendrimer is robust andshould lead to a membrane that exhibits significant chemical stabilitytoward chemical attack within the highly corrosive fuel cellenvironment. Additional protection against adventitious peroxideradicals generated in situ within a fuel cell results from the compactglobular structure each dendrimer maintains in a high ionic strengthmedium, i.e., pH≦1.0. Because this type of dendrimer issurface-functionalized with 32 carboxylic acid groups, membranesfabricated using these dendrimers as building blocks will possess lowacid equivalent weights, estimated to range between about 150 and about500. In some embodiments the EW will be between about 200 and about 400,and in other embodiments between about 240 and about 250, which is astriking improvement compared to current membranes (1100 to 800 forNAFION (perfluorosulfonic acid polymer) and Dow membranes,respectively). The large decrease in the EW results in markedly higherproton-conductivities and leads to a more hydrophilic structure. Thegreater hydrophilicity leads to a structure that retains some water attemperatures above 100° C. and leads to improved performance at thesetemperatures compared to current membranes. Moreover, it is believedthat proton or hydronium ion transport proceeds on the surface of thespherical or globular dendrimers, much like grain boundary diffusion insolids. This drastically decreases the dependence of the ionic transporton the water content. Due to the higher thermal stability and ionictransport mechanism, these membranes are useful in higher temperatureregimes (100-200° C.).

In one embodiment, the dendrimers are fluorinated dendrimers. In thisembodiment dendrimers, including dendrimers with fluorinatedsubstituents just at the outer edges of the dendrimer, are used toconstruct the membrane. Fluorinated dendrimers can be synthesized usingfluorinated analogues to the precursor molecules used to make theFrechét-type dendrimers. To provide a perfluorinated dendrimer,perfluorinated analogs of the starting materials for the unfluorinateddendrimers would be used throughout the synthesis. Methods for makingthese dendrimers are well known in the art. For example, a descriptionof a method for making such dendrimers can be found in Miller, T. M.,Neenan, T. X., Zayas, R., Bair, H. E., J. Am. Chem. Soc. (1992) 114,1018, which is herein incorporated by reference. Briefly, to obtain adendrimer with fluorine only at the outer surface, two equivalents ofmethyl-p-bromomethyl(tetrafluoro)benzoate can be reacted with3,5-dihydroxybenzyl alcohol in the presence of 18-crown-6 and potassiumcarbonate in refluxing acetone. The resulting first generation alcoholwould have fluorinated aryl groups at the outer surface which would becarried along throughout the synthesis of higher generation dendrimers.Likewise, to provide further steric protection,2,6-trifluoromethyl-bromomethylbenzoate can be utilized in place ofp-bromomethylbenzoate in the first step of the synthetic procedure. Thiswill provide a generation one dendrimer with perfluoromethyl groups atthe outer surface. The electronic and steric effects of the fluorinatedphenyl substituents and perfluoromethyl substituents would impartgreater chemical stability towards peroxide radicals compared to theunfluorinated dendrimer with minimal fluorination.

Other types of dendrimers are also suitable for use in the constructionof the membranes of the invention. These include those utilizing directcarbon to carbon linkages between generations of dendrimer rather thanether linkages, or linkages utilizing functional groups containingnitrogen, oxygen, silicon, phosphorous or transition metal heteroatoms,as well as systems based on repeat units utilizing these heteroatoms.Examples include dendrimers formed from multi-substituted arylcompounds, siloxanes, carbosilanes, amines, phosphines and phosphineoxide compounds. Methods for making such dendrimers can be found in F.J. Stoddart and T. Welton, Polyhedron (1999) 18, 3575; H. Lang and B.Luhmann, Adv. Mater. (2001) 13(20), 1523, and K. Takagi, T. Hattori, H.Kunisada, and Y. Yuki, J. of Polym. Sci. Part A-Polym. Chem. (2000)38(24), 4385, which are herein incorporated by reference. Theheteroatoms can be used to tune the geometry by supplying different bondangles and bond distances, and can help tailor the electronic structureto increase chemical stability and/or increase the acidity of the acidgroups to increase the conductivity. Examples of multi-substituted arylcompounds include 1,3,5-trihydroxyaryl compounds, 1,3,5-trialkoxyarylcompounds, and 3,5-disubstitutedaryl compounds. Methods for making thesedendrimers can be found in Miller, T. M., Neenan, T. X., Zayas, R., andBair, H. E., J. Am. Chem. Soc. (1992) 114, 1018, and T. M. Miller et.al. ACS Polymer Preprints, (August 1991) 32, 627, which are hereinincorporated by reference.

In various embodiments of the invention, dendrimers having phosphonic orsulfonic acid groups or derivatives of sulfonic acids and phosphonicacids as terminal groups, are used instead of dendrimers havingcarboxylic acid terminal groups. Frechét-type dendrimers with sulfonicacid functionality, may be produced in a controlled manner by replacingthe methyl-p-bromomethylbenzoate starting material, described above,with either an amide-protected, sulfonic acid or thiolbromomethylbenzoate. Both4-(halomethyl)-1-(N,N-dimethyl-sulfonamido)benzene and4-(halomethyl)benzenethiol (halogen=Cl, Br, I) are the chemicalanalogues to methyl 4-(bromomethyl)benzoate, which is converted to thecarboxylic acid terminal groups to produce carboxylic acid terminateddendrimers. A sulfonic acid derivative is made by reacting twoequivalents of 4-bromomethyl-1-(N,N-Dimethylsulfonamido)benzene with 1equivalent of 3,5-dihydroxybenzyl alcohol in the presence of 18-crown-6and potassium carbonate in refluxing acetone. The resulting firstgeneration alcohol is converted to the bromide by reaction with carbontetrabromide/triphenylphosphine. Further generations are prepared in amanner similar to that described above for the production of thecarboxylic acid terminated dendrimers. In another route, 2 equivalentsof 4-bromomethyl-1-(N,N-dimethylsulfonamido)-benzene can be reacted withone equivalent of the 3,5 dihydroxymethyl ester of benzoic acid and fourequivalents of diethyl azodicarboxylate and triphenylphosphine in THF toprovide a first generation methylester. This methylester is then reducedwith LiAlH₄ to produce the first generation alcohol, and the procedurerepeated for higher generation dendrimers. The sulfonic acid dendrimercan then be obtained by an acid catalyzed hydrolysis of the sulfonamidedendrimer. Alternatively, the sulfonic acid functionality can be addedby converting the terminal groups on the dendrimer to sulfonic acidgroups as was performed by Gong, et. al. in A. Gong, Q. Fan, Y. Chen, H.Liu, C. Chen, and F. Xi, J. of Mol. Catal. A: Chem. (2000) 159, 225,which is herein incorporated by reference.

Once the dendrimers have been produced they are covalently linked into adendrimer network. This can be accomplished by a variety of linkingreactions well known in the art. Regardless of the specific couplingmethod selected, this strategy offers the advantage of directing theassembly of the dendrimers as macroscopic building blocks for an ionomermembrane, with implications for controlling both pore size and themicrostructure of the membrane on a molecular level. Water containingnetworks can be produced by including water into the reaction mixtures.Alternatively, the networks may be exposed to a humidified air fromwhich they will pick up water to form a water containing network.

Dendrimers having acid terminal groups can be linked through amido orimido bridges under various reaction conditions, using amide or imidegroup-containing linking compounds. These include poly- anddi-functional amines which can react with the acid groups on at leasttwo different dendrimers to form a network polymer that can be cast intoa membrane. This approach leads to a random covalent network. Suitablediamine linking compounds include diaminoaryl compounds, diaminoalkanecompounds, and diaminoalkene compounds. Particularly preferred diaminesinclude 1,4-diaminobenzene and 1,2-diaminoethane. One route to thesediamine bridged species is to convert the carboxylic acid terminalgroups of the dendrimers into acid chloride groups, followed by reactingthe acid chlorides with the diamine. The carboxylic acid groups can beconverted to the acid chloride by reaction with SOCl₂. The diamine thenreacts with the terminal acid chloride to eliminate HCl and form thediamine bridged dendrimers. FIG. 2 shows a representation of a portionof a membrane according to the present invention comprised of sphericaldendrimers covalently linked with diamine compounds.

Alternatively, the dendrimers may be linked by an alcoholgroup-containing compound. Suitable alcohol group-containing linkingcompounds include 1,4-dihydroxy benzene, 4,4′-dihydroxybiphenyl,ethylene glycol, polyethylene glycol, and the like.

In another embodiment of the invention, membranes can be composed ofdendrimers having halide or hydroxyl terminal groups substituted for aportion of the acid functional terminal groups. The halide or hydroxylterminal groups then serve as reacting points that are used tocovalently link the dendrimers through cross-coupling reactions. Forexample a dendrimer having a bromide terminal group may be formed asfollows: 1 equivalent of 4-chloro-1-bromomethyl benzene can be reactedwith 1 equivalent of 4-bromomethyl-1-(N,N-dimethyl-sulfonamido)benzeneand 1 equivalent of 3,5-dihydroxybenzyl alcohol in the presence of18-crown-6 and potassium carbonate in refluxing acetone. This mixedfirst generation dendrimer can then be reacted with 1 equivalent of afirst generation alcohol and 1 equivalent of dihydroxy benzyl alcohol inthe presence of 18-crown-6 and potassium carbonate to provide a secondgeneration dendrimer with 1 halide group. This can be bromated andcarried through to higher generations in a manner similar to thosepreviously described. Similar methodology was used to prepare a Brfunctionalized dendrimer by Z. Bo, A. Schafer, P. Franke, and A. D.Schluter, in Org. Lett. (2000) 2, 1645, which is herein incorporated byreference.

Dendrimers having both acid and halide or hydroxyl terminal groups,including Frechét-type dendrons based on 3,5-dihydroxybenzyl alcoholwith carboxylic acid and halide, (e.g., chloride, bromide and iodide)terminal groups, can be selectively cross-coupled to form stablecovalent links between the halide-modified terminal groups on thedendrimers. Such cross-coupling reactions are well known in the art andinclude, but are not limited to, Suzuki coupling (reaction ofarylboronic acids with aryl halides catalyzed by Pd complexes such asPd(PPh₃)₄), Stille coupling (reaction of a trialkyl tin complex with anaryl or allyl halide to eliminate a trialkyl tin halide and form acarbon-carbon bond), Kumada coupling (reaction of a 3,5-dibromo arylspecies with 2 equivalents of an aryl-Grignard reagent in the presenceof Ni(II) to provide an aryl bridge between the aryl components of theGrignard reagent), Ulmann coupling (reaction of an aryl halide with asecond aryl halide or alcohol in the presence of Cu to produce a diarylspecies or diaryl ether species), and related coupling reactions for theformation of specific covalent biphenyl bridges between the arylpolyether groups of dendrimers. For example,palladium-imidazol-2-ylidene complexes can be used as catalysts for thefacile and efficient Suzuki cross-coupling between unactivated arylchlorides and aryl diboronic acids. Efficient cross-coupling also occursbetween unactivated aryl chlorides and aryl Grignard reagents like1,4-bis(bromomagnesium)benzene using a palladium-imidazolium chloridecatalyst system in the Kumada reaction. This chemistry can be furtherextended to include Stille cross-coupling between aryl chlorides andaryl distannyl reagents like 1,4-bis(tributyltin)benzene using atris(t-butyl)phosphine-palladium catalyst system. Diaryl ether bridges,as opposed to biphenyl bridges, between dendrimers can be readilysynthesized by either a copper- or a palladium-catalyzed Ulmann diarylether synthesis between aryl bromides or iodides and bisphenols like1,4-dihydroxybenzene.

Conversely, if the dendrimers are terminated with carboxylic acid andhydroxyl terminal groups, then the dendrimers can be crosslinked viaUlmann diaryl ether synthesis, facilitated using 1,4-dihalobenzenes.Methods for making these compounds, including methods similar to thosedescribed above to forming halide terminated dendrimers, are well knownin the art. A method for making hydroxyl terminated dendrimers isdescribed by C. Kim, S. Son, and B. Kim, in J. Organomet. Chem. (1999)588, 1, and H. Ihre, A. Hult, and J. M. J. Frechét, I. Gitsov, inMacromolecules (1998) 31, 4061, which are herein incorporated byreference.

Membranes composed of dendronized copolymers are also contemplated bythe present invention. These membranes, which are made from dendrimershaving both acid terminal groups and polymerizable terminal groups, arecovalently linked by polymerization reactions between the polymerizableterminal groups on the dendrimers. Polymerizable terminal groupsinclude, but are not limited to, vinyl, styrene, methacrylates,urethanes, amides, imides, thiophenes, and aryl alkynes. For example,Frechét-type dendrons based on 3,5-dihydroxy-benzyl alcohol or relateddi- and trihydroxybenzyl alcohols can be reacted with 4-bromostyrene ina copper or palladium catalyzed Ulmann ether synthesis to generatemacromonomers that in turn can be polymerized like polystyrene togenerate dendrimers that contain styrene groups at some of the termini.The styrene groups in turn can be polymerized to generate dendronizedcopolymers.

Once the dendrimers are covalently linked either through the use of across-linking compounds, through cross-coupling reactions, or throughcopolymerization to form a polymer network, the polymer network is castinto a membrane. In one embodiment the membranes have a thickness ofbetween about 0.02 and about 0.2 millimeters. The dendrimers may be castinto membranes by methods well known in the art. The suitability of agiven method will depend upon the properties of the dendrimer. Examplesof suitable methods include, but are not limited to, solution casting,melt casting, and spin casting.

One aspect of the invention provides dendrimer-ionic liquid compositemembranes. In these membranes, ionic liquids, such as substitutedimidazole salts, are imbibed in the membrane as a functional replacementof the water present in the membranes. High-boiling point ionic liquidscan function as nonaqueous, mechanistic alternatives to water for protonconductivity through the membranes, thus allowing for high temperatureoperation without loss of the conducting liquid in the pores.Dendrimer-ionic liquid composites that do not depend upon waterhydration for proton conductivity can produce protons without methanolcrossover in temperature regimes greater than 100° C. Examples of ionicliquids are substituted imidazole salts, such as1-n-butyl-3-methylimidazolium and 1-ethyl-3-methylimidazolium. Thesesalts can be trapped in any of the dendrimer membranes described above(e.g. the Frechét-type dendrimer linked with amido bridges) by includingthem in the reaction mixture during the linking process.

Any membranes fabricated using the type of dendrimers described above orrelated dendrimers having discrete and controllable polymericarchitectures are important to the construction of a fuel cell. In itssimplest construction, a fuel cell is made up of a thin, solid polymerelectrolyte that is pressed between two electrocatalyst layers, an anodeand a cathode layer. This is a membrane electrode assembly (MEA), andthe membrane functions as a protonic conductor, an electrical insulatorand a barrier separating the reactant feed streams to the anode andcathode. Electrical current flows from the anode to the cathode,completing useful mechanical work as it does so. This is largely truewhether the fuel cell oxidizes hydrogen generated from the reforming ofhydrocarbons or oxidizes methanol directly in a direct methanol fuelcell. The use of these dendrimers as nano- or mesoscale objects tofabricate a larger membrane superstructure is readily possible, with theparticular advantage that the thickness of the active layers of themembrane can be minimized within the MEA. A fuel cell fabricated usingsuch dendrimers will be able to operate at higher temperatures (>100°C.) due to better water management within the membrane. Methanolcrossover will also be reduced or eliminated when such a membrane isused in a direct methanol fuel cell. Each individual fuel cell can beplaced in series to form a stack, and several stacks together generatethe electrical energy necessary for mobile and portable powerapplications. In particular, polymer electrolyte membranes may be usefulfor transportation applications. The operating parameters of the fuelcell are determined by the fuel. As previously discussed, the fuel canbe hydrogen, methanol, or reformate (a gas containing hydrogen, carbonmonoxide, carbon dioxide and water prepared from hydrocarbon feeds)which will have to be cleaned up before entering the fuel cell.Consequently, a power plant using a direct methanol fuel cell is simplein construction; reforming hydrocarbon fuels to generate hydrogen ondemand currently requires much more ancillary equipment, e.g., areformer reactor, a sulfur removal catalytic reactor, a high and a lowtemperature water-gas shift catalytic reactor, and a catalytic reactorfor the preferential oxidation of CO in H₂-rich feed streams.

As will be understood by one skilled in the art, for any and allpurposes, particularly in terms of providing a written description, allranges disclosed herein also encompass any and all possible subrangesand combinations of subranges thereof. Any listed range can be easilyrecognized as sufficiently describing and enabling the same range beingbroken down into at least equal halves, thirds, quarters, fifths,tenths, etc. As a non-limiting example, each range discussed herein canbe readily broken down into a lower third, middle third and upper third,etc. As will also be understood by one skilled in the art all languagesuch as “up to,” “at least,” “greater than,” “less than,” and the likeinclude the number recited and refer to ranges which can be subsequentlybroken down into subranges as discussed above.

The invention is described in more detail in the following non-limitingexamples.

EXAMPLE 1 Production of a Membrane Made from Diamine-Linked FrechéT-TypeDendrimers

Frechét-type dendrimers are formed as described by Hawker et al. in J.Chem. Soc. Perkin Trans. (1993) 1, 1287. Such dendrimers are formed byreacting two equivalents of methyl-p-bromomethylbenzoate with3,5-dihydroxybenzyl alcohol in the presence of 18-crown-6 and potassiumcarbonate in refluxing tetrahydrofuran (THF). The resulting firstgeneration (G1) alcohol is converted to the bromide in an 83 percentyield by reaction with carbon tetrabromide/triphenylphosphine. Reactionof two equivalents of this bromide with 3,5-dihydroxybenzyl alcoholleads to the second generation dendrimer in a 91 percent yield. Thesesteps are repeated to get to generation 3, then 4 dendrimers in yieldsof about 85 percent. At generation 4, two equivalents of brominatedgeneration 4 dendrimer are reacted with 4,4′-dihydroxybiphenyl inrefluxing THF to provide a [G4]-O—C₆H₅—O-[G4] macromolecule in an 81percent yield after purificatin by flash chromatography. The methylester groups are then converted by reaction with excess potassiumhydroxide to provide the carboxy terminated dendrimer.

After the dendrimers are formed they are covalently linked to produce anetwork structure. This is accomplished by first converting a portion ofthe carboxylic acid terminal groups on the dendrimers into acid chloridegroups by reacting 8.5 grams of the [G4]-O—C₆H₅—C₆H₅—O-[G4]macromolecule with 0.9 grams of SOCl₂ in refluxing THF. Approximately0.5 grams of 1,2-diaminoethane is then added to the reaction mixture andallowed to react with the acid chloride-terminated dendrimers overnightto form a network structure of diamine bridged dendrimers.

Finally, the dendrimer network is cast into a membrane. This isaccomplished by dissolving 2-5 grams of the dendrimer network (dependingon the film thickness desired) in 100 ml of THF, pouring the solutioninto a petri dish with a diameter of 2-4″, and slowly evaporating thesolvent. Initial evaporation is performed in air, followed by vacuumevaporation at about 50° C. Finally, the film is removed and pressedbetween two teflon sheets at around 80-100° C. and 500-1000 psi.

EXAMPLE 2 Production of a Membrane Made from Diamine Linked FrechéT-TypeDendrimers

Frechét-type dendrimers are formed as described by Forier et al. inTetrahedron (1999) 55, 9829. Such dendrimers are formed by adding 6.9grams of methanesulphonyl chloride to 4,8 grams of a G-1 alcohol, whichis produced as described in Example 1 above, and 7.6 grams of triethylamine in 50 ml of dry dichloromethane at −10° C. under a nitrogenatmosphere. The mixture is stirred for one hour then poured over amixture of 100 ml of crushed ice and 10 ml of concentrated hydrochloricacid (HCl). The organic layer is then separated, washed with an NaHCO₃solution, dried over MgSO₄, and evaporated. The product isrecrystallized from diethyl ether to give a [G1]-OSO₂CH₃ product at an87 percent yield.

Higher generation dendrimers are prepared in a similar manner.Specifically, the [G2] alcohol is prepared from the [G1]-OSO₂CH₃ byrefluxing in an acetone solution (70 ml) containing 7 grams of the[G1]-OSO₂CH₃, 0.82 grams of 3,5-dihydroxybenzyl alcohol, 4.9 grams ofK₂CO₃ and 0.1 grams of 18-crown-6 overnight under a nitrogen atmosphere.The reaction mixture is poured into 300 ml of water and extracted withdichloromethane. The combined extracts are dried over MgSO₄, thenevaporated and the residue chromatographed on silica gel withdichloromethane/diethyl ether (20:1) to give the [G2]-OH in 87 percentyield (3.8 g). The [G2]-OH is converted to the [G2]-OSO₂CH₃ by adding2.5 grams of methanesulphonyl chloride to a 50 ml dichloromethanesolution containing 3.9 grams of the [G2]-OH and 2.7 grams of triethylammine. The product is recrystallized from diethyl ether to give the[G2]-OSO₂CH₃ in 87 percent yield (3.8 g). From this product, the [G3]-OHis prepared. A mixture containing 2.5 grams of the [G2]-OSO₂CH₃, 0.14grams of 3,5-dihydroxybenzyl alcohol, 0.85 grams of K₂CO₃ and 0.1 gramsof 18-crown-6 is refluxed in 40 ml of dry acetone overnight. The residueis chromatographed on silica gel with dichloromethane/diethyl ether(25:1) to give the product in 85 percent yield. This is converted to the[G3]-OSO₂CH₃ by adding 0.4 grams of triethyl ammine and 0.4 grams ofmethanesulphonyl chloride to 1.2 grams of the [G3]-OH in 20 ml of drydichloromethane. The crude material is chromatographed to provide the[G3]-OSO₂CH₃ in 85 percent yield. This product is then converted to the[G4]-OH. A mixture containing 1.5 grams of [G3]-OSO₂CH₃, 0.042 grams ofdihydroxybenzyl alcohol, 0.85 grams of K₂CO₃ and 0.1 grams of 18-crown-6is refluxed in 40 ml of dry acetone. The residue is chromatographed onsilica gel with dichlorometrhane/diethyl ether to give the [G4]-OH in an84 -percent yield. (0.82 g) The [G4]-OSO₂CH₃ is prepared by adding 0.11grams of methanesulphonyl chloride to a 20 ml dichloromethane solutioncontaining 0.8 grams of the [G4]-OH and 0.13 grams of triethyl amine.The product is recrystallized from diethyl ether to give the[G4]-OSO₂CH₃ in 87 percent yield (0.71 g).

To form a spherical dendrimer, the [G4]-OSO₂CH₃ is reacted with4,4′-biphenol. In refluxing acetone solution (70 ml), 3 grams of the[G4]-OSO₂CH₃ is reacted with 0.055 grams of 4,4′-biphenol, 0.18 grams ofK₂CO₃, and 0.1 grams of 18-crown-6 overnight under a nitrogenatmosphere. The residue is chromatographed on silica gel withdichloromethane/diethyl ether (25:1) to give the [G4]-O—C₆H₅—C₆H₅—O-[G4]complex in 85 percent yield (1.7 g). A THF solution containing 0.7 gramsof this dendrimer in 50 ml of THF is added dropwise to a suspension ofcontaining 0.36 grams of LiAlH₄ in 20 ml of THF. After refluxing forapproximately 2 hours, the solution is treated with aqueous NaOH (1 M,15 ml), filtered, and evaporated. The residue is chromatographed on asilica gel column with dichloromethane as the eluent to produce thecarboxylic acid derivative in a yield of approximately 85 percent. Thisproduct is refluxed with 0.022 grams of SOCl₂ in THF, then 0.045 gramsof 1,2-diaminoethane is added to the reaction mixture and allowed toreact with the acid chloride-terminated dendrimers for approximately 2hours to form a network structure of diamine bridged dendrimers. Thesolution is concentrated and poured onto a plate and the solvent isevaporated to provide a solid.

Finally, the dendrimer network obtained above is cast into a membrane.This is accomplished by placing 1-2 grams of the dendrimer networkpolymer between two teflon coated aluminum plattens. The plattens areplaced in a Carver Laboratory Press (Model C) preheated to 200-220° C.After the material softens, the plattens are subjected to a pressure of2,500-5,000 psi for approximately 1 minute. The pressure is released andthe aluminum platens removed from the press and allowed to cool. Thedendrimer membrane can then be carefully removed form between theplattens.

While preferred embodiments have been illustrated and described, itshould be understood that changes and modifications can be made thereinin accordance with ordinary skill in the art without departing from theinvention in its broader aspects as defined in the following claims.

1. A method for preparing an ion-conducting membrane comprising thesteps of: (a) reacting a linking compound with a plurality ofdendrimers, having acid functional terminal groups, to form a covalentlylinked network polymer; and (b) casting the network polymer into anion-conducting membrane, wherein the ion-conducting membrane has an acidequivalent weight between about 150 and about
 500. 2. The method ofclaim 1, wherein the acid functional terminal groups are selected fromthe group consisting of carboxylic acids, carboxylic acid derivatives,sulfonic acid functional terminal groups, sulfonic acid derivatives,phosphonic acid functional terminal groups, and phosphonic acidderivatives.
 3. The method of claim 1, wherein the acid functionalterminal group is a carboxylic acid.
 4. The method of claim 1, whereinthe linking compound is selected from the group consisting ofdi-functional amines, polyfunctional amines, and alcoholgroup-containing linking compounds.
 5. The method of claim 4, whereinthe polyfunctional amine is a diaminoaryl compound, diaminoalkane, ordiaminoalkene.
 6. The method of claim 1, further comprises adding wateror exposing the covalently linked network polymer to humidified air. 7.The method of claim 1, wherein the ion conducting membrane furthercomprises an ionic liquid.
 8. The method of claim 1, wherein thedendrimers are Frechét type dendrimers.
 9. The method of claim 1,wherein the ion conducting membrane is thermally stable at temperaturesbetween about 100° C. and about 200° C.
 10. The method of claim 1,wherein the ion conducting membrane has an acid equivalent weightbetween about 200 and about
 400. 11. The method of claim 1, wherein atleast some of the dendrimers are fluorinated.
 12. The method of claim 1,wherein the acid functional terminal group is a methyl ester and themethod further comprises converting the methyl ester to a carboxylicacid.
 13. A method for preparing an ion-conducting membrane comprisingthe steps of: (a) cross-coupling a plurality of dendrimers having acidfunctional terminal groups and terminal groups capable of undergoingcross-coupling reactions to form a covalently linked network polymer;and (b) casting the network polymer into an ion-conducting membrane,wherein the ion-conducting membrane has an acid equivalent weight ofbetween about 150 and about
 500. 14. The method of claim 13 wherein theterminal groups capable of undergoing cross-coupling reactions areselected from the group consisting of halide terminal groups andhydroxyl terminal groups.
 15. The method of claim 13, wherein thecross-coupling occurs through cross-coupling reactions selected from thegroup consisting of Susuki coupling, Stille coupling, Kumada coupling,and Ulmann coupling.
 16. The method of claim 13, wherein the acidfunctional terminal group is a carboxylic acid.
 17. A method forpreparing an ion-conducting membrane comprising the steps of: (a)copolymerizing a plurality of dendrimers having acid functional terminalgroups and terminal groups capable of undergoing copolymerizationreactions to form a covalently linked network polymer; and (b) castingthe network polymer into an ion-conducting membrane, wherein theion-conducting membrane has an acid equivalent weight between about 150and about
 500. 18. The method of claim 17, wherein the terminal groupsare selected from vinyls, styrenes, methacrylates, urethanes, amides,imides, thiophenes, and aryl alkynes.
 19. The method of claim 17,wherein the acid functional terminal group is a carboxylic acid.
 20. Themethod of claim 17, wherein the ion conducting membrane has an acidequivalent weight between about 200 and about 400.